A direct Z-scheme BiOBr/TzDa COF heterojunction photocatalyst with enhanced performance on visible-light driven removal of organic dye and Cr(VI)

Design of efficient Z-scheme heterojunction photocatalysts is important and yet challenging for purifying wastewater with solar energy. Herein, a new 2D/2D Z-scheme heterostructure photocatalyst of BiOBr/TzDa covalent organic framework (COF) composite was obtained by modification of 2D BiOBr with 2D TzDa COF via a facile hydrothermal method. The photocatalytic activity of BiOBr/TzDa COF (BTDC) under simulated visible light was studied by degradation of single Rhodamine B (RhB) (10 ppm) and reduction of single Cr(VI) (10 ppm), as well as for treating the RhB/Cr(VI) mixture. RhB could be efficaciously degraded by BTDC within a wide pH range of 2.1–8.1. Nearly complete reduction of Cr(VI) at pH 2.1 could also be achieved after irradiation for 90 min. Moreover, much higher performance of BTDC was found for simultaneous removal of RhB and Cr(VI) from a binary mixture solution than for single pollutant system due to the synergistic effect within the RhB/Cr(VI)/BTDC system. 97% of total RhB and Cr(VI) in the mixture could be removed in 20 min and 40 min, respectively. The promoted efficiency of the stable and recyclable BTDC could be mainly attributed to accelerated charge carriers separation and transport, enhanced visible light absorption as well as increased specific surface area. Photocatalytic mechanism with Z-scheme charge transfer in BTDC was suggested based on systematic studies of band structures calculation, radical quenching experiments and ESR analyses. This study may provide a strategy in constructing COF based heterojunctions for purification of wastewaters containing mixed heavy metal ions and organic pollutants.